So... I have reasons (namely the Beast of the Sea) for creating an underwater environment as a player. Of course, I looked it up and found nothing discussing the topic. So the question is; would there be a way to make an underwater environment at all? (Of course, this would definitely be a last-ditch effort in case all else fails, as to prevent my party members from suffering the consequences. And in case you haven't notice, I really want a Beast of the Sea.)

When you say "create an underwater environment", are you talking about creating one as a player wherever you happen to be at the time? Something like conjuring enough water to fill the underground cavern you are currently exploring?

Quote from Sequilonis >>

When you say "create an underwater environment", are you talking about creating one as a player wherever you happen to be at the time? Something like conjuring enough water to fill the underground cavern you are currently exploring?

 Creating one as a player, yes.

Quote from Platyboss >>

So... I have reasons (namely the Beast of the Sea) for creating an underwater environment as a player. Of course, I looked it up and found nothing discussing the topic. So the question is; would there be a way to make an underwater environment at all? (Of course, this would definitely be a last-ditch effort in case all else fails, as to prevent my party members from suffering the consequences. And in case you haven't notice, I really want a Beast of the Sea.)

 Basically, you've got Watery Sphere, and Wall of Water, or carrying around a barrel full of water on your back.

Create a Gate into the Elemental Plane of Water, if your DM will allow it? Technically the Gate spell doesn't say anything about how stuff passes through it, if at all. 

I’m assuming you’re talking about the bast master ranger’s beast of the sea. From my view point you shouldn’t be trying to carry around a water environment as you would have to fit the entire adventure into it to make the beast of the sea more useful than the other 2 beasts. I would talk to the DM about putting parts of the adventures along bodies of water (lakes, rivers, shorelines, etc) where selecting a beast of the sea could make sense. Of course you need to be L6+ and have the water breathing spell to get the best usage.

Quote from Wi1dBi11 >>

I’m assuming you’re talking about the bast master ranger’s beast of the sea. From my view point you shouldn’t be trying to carry around a water environment as you would have to fit the entire adventure into it to make the beast of the sea more useful than the other 2 beasts. I would talk to the DM about putting parts of the adventures along bodies of water (lakes, rivers, shorelines, etc) where selecting a beast of the sea could make sense. Of course you need to be L6+ and have the water breathing spell to get the best usage.

The Beast Master Ranger's Beast of the Sea can breathe Air and Water.

So if you don't care about its comfort, and only care about its game-mechanical effects, you can just push it around in a wheelbarrow, or cart it around on Tenser's Floating Disk.  If maneuvering in combat in needed,  Longstrider can help a bit, Fly can help a lot more. Unfortunately by default, it doesn't have enough Move to benefit from Jump on land without augmentation.

Assistance Option:  Have someone with the Grappler feat drag it around.

Comedy option: Define the Beast's design to include human-like feet. (It's a literal Sea Monkey!) and give it Boots of Striding and Springing.